chore(deps): update dependency google-cloud-texttospeech to v2.4.0 (#166

)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-texttospeech](https://togithub.com/googleapis/python-texttospeech) | `==2.3.0` -> `==2.4.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-texttospeech/2.4.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-texttospeech/2.4.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-texttospeech/2.4.0/compatibility-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-texttospeech/2.4.0/confidence-slim/2.3.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-texttospeech</summary>

### [`v2.4.0`](https://togithub.com/googleapis/python-texttospeech/blob/master/CHANGELOG.md#&#8203;240-httpswwwgithubcomgoogleapispython-texttospeechcomparev230v240-2021-05-28)

[Compare Source](https://togithub.com/googleapis/python-texttospeech/compare/v2.3.0...v2.4.0)

##### Features

-   support self-signed JWT flow for service accounts ([8a08836](https://www.github.com/googleapis/python-texttospeech/commit/8a08836487c1b7e4e58d3c07a4e26005d40793f0))

##### Bug Fixes

-   add async client to %name\_%version/init.py ([8a08836](https://www.github.com/googleapis/python-texttospeech/commit/8a08836487c1b7e4e58d3c07a4e26005d40793f0))

</details>

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translation/samples/snippets/hybrid_glossaries/requirements.txt

@@ -1,3 +1,3 @@
 google-cloud-translate==3.2.0
 google-cloud-vision==2.3.1
-google-cloud-texttospeech==2.3.0
+google-cloud-texttospeech==2.4.0